When the engines of an aircraft are not in use electrical power, hydraulic power and bleed air power are provided by on board APUs. These APUs include turbines which directly provide bleed air and indirectly provide electrical and hydraulic power through conversion of turbine rotary shaft power by use of electrical generators and pumps. Often the conversion of rotary shaft power from APUs includes use of gearboxes to match the rotational speed and torque output of the APUs to the requirements of generators and pumps. Clutches are often used between the APUs and gearboxes.
An example describing use of clutches between aircraft turbines and gears is set out in U.S. Pat. No. 4,799,354 issued to Midgley. Turbines described here are connected to propellers, or fans, through systems of gears with clutches located between portions of the gear systems. The gear systems include two sets of gears, one set of gears directly connected to turbine shafts, and the other sets of gears directly connected to propellers or fans. Clutches are used between the two sets of gears, and it is asserted these clutches disengage the mechanical loads of the propellers, or fans, from the turbines when starting the turbines. As described, only propellers, or fans, are disconnected and no reference is made to APU turbines.
Another example describing use of clutches is set out in U.S. Pat. No. 4,077,202 to Schutze. The described system includes an aircraft engine, an auxiliary gas turbine for producing rotational power as well as pressurized gas, and an air turbine to provide compressed gas, i.e., pneumatic power, to the auxiliary gas turbine. The shafts of the aircraft engine and the auxiliary gas turbine are connected by clutches to a gearbox. These clutches are preferably override clutches. Connected, without clutches, to the gearbox are auxiliary equipment such as pumps and electric generators. As connected to the gearbox by override clutches the aircraft engine and auxiliary gas turbine can turn the shafts of the pumps and electric generators, but the shafts of the aircraft engine and auxiliary gas turbine cannot be turned by the gears in the gearbox because the override clutches only permit one way drive. With respect to the air turbine, a override clutch connects the aircraft engine to the air turbine shaft so the aircraft engine can be driven by the air turbine shaft but not vice versa.
The U.S. Pat. No. 4,864,812 issued to Rogers et al. describes still another example of clutches used with turbines on aircraft. Here, use of both APUs and emergency power units (EPUs) on a single aircraft is addressed. The APU provides electrical and hydraulic power and bleed air when aircraft engines are not in use, and the EPU is capable of quickly responding to unexpected aircraft power failures even at high altitudes to provide sufficient electrical and hydraulic power so a pilot can regain aircraft control. The system described includes a mechanical gearbox connected to an electrical generator, hydraulic pump and other accessory equipment to be driven by either the APU or EPU. A clutch is connected between the gearbox and APU, and a second clutch is connected between the gearbox and EPU. In normal operation, the clutch between the APU and gearbox is engaged and the clutch between the EPU and gearbox is disengaged. If there is a system failure requiring emergency power the clutch between the APU and gearbox is disengaged and the clutch between the EPU and gearbox is engaged.
Ignored by the above described systems is reduction of mechanical loads applied to APU output shafts during APU turbine start-up. Starting of APUs on aircraft to drive pumps and electric generators is not facilitated by use of clutches to: (1) disengage propellers, or fans, during starting of aircraft main engines; (2) selectively connect aircraft engines or auxiliary gas turbines to gearboxes used to drive pumps and electric generators; or (3) selectively connect APUs or EPUs to gearboxes used to drive pumps and electric generators. Without reductions in mechanical loads applied to APU output shafts, the on board systems used to start APUs must be sized in both volume and weight to start APU turbines and concurrently drive mechanical loads through gearboxes.